This invention relates to image transmission or storage systems, such as high definition television (HDTV), digital NTSC, and other advanced television systems.
The desired resolution goal for terrestrial HDTV broadcasting within a single 6 MHz channel is approximately 1000 lines (e.g., 960 lines or 1080 lines) with progressive scanning (PS) at 60 frames/sec. Unfortunately, presently available technology is not capable of meeting that goal, and thus it is expected that the transmission formats selected for the HDTV standard will either have fewer lines, fewer frames/sec, or use interlaced (IS) scanning instead of progressive scanning (PS). For example, one proposed set of standard formats is shown in Table 1:
TABLE 1 ______________________________________ One Proposed Set Of HDTV Standard Transmission Formats Scan Spatial Resolution Frame Rate Format ______________________________________ 720 .times. 1280 pixels 60 frames/sec PS 720 .times. 1280 pixels 30 frames/sec PS 720 .times. 1280 pixels 24 frames/sec PS 1080 .times. 1920 pixels 30 frames/sec PS 1080 .times. 1920 pixels 24 frames/sec PS 1080 .times. 1920 pixels 60 fields/sec IS ______________________________________
The reason why the desired 1080 lines, 60 frames/sec, and PS scan format may not be included in the HDTV standard is that to transmit such a format using today's video compression technology requires more bits per second than can be accommodated in the available channel bandwidth. Typical HDTV images are expected to have a 16:9 aspect ratio with square pixels, and thus the total number of pixels/sec in a format having 1080 lines, 1920 pixels/line, and PS at 60 frames/sec is approximately 125 Mpixels/sec. The technologies such as QAM and VSB proposed for HDTV transmission over the air provide approximately 27 Mbits/sec (5 bits/Hz for 32-QAM with 5.4 MHz of usable bandwidth) with reasonable coverage area. Excluding the bit capacity needed for error correction, audio, auxiliary data, etc., the total number of bits available for encoding the video data is around 18-20 Mbits/sec or about 0.14 to 0.16 bits/pixel, which is too low for the proposed video compression methods (motion-compensated DCT) to operate successfully without significant video quality degradation. By adopting formats with fewer overall pixels/sec, e.g., those in Table 1, it is presently possible to achieve acceptable video quality.
This presents a difficult problem, for by adopting standard transmission formats now, the benefits of future improvements in video compression methods could be foreclosed to consumers, as it is highly unlikely that the formats adopted today for HDTV will be changed anytime soon. Just as the NTSC format remained the standard for decades, it can be expected that the new HDTV standard will be in place for a long period of time.
One possible solution would be to include one or more higher resolution formats within the HDTV standard (e.g., a 1080 line, 60 frame/sec, PS format), and reserve its use for a future time when video compression techniques would make it possible. But that would be prohibitively costly, as all receivers would have to be built with the greater processor speeds and memory capacities necessary to handle the higher resolution formats, and no immediate advantage would be achieved.
Hurst, "Interlace to Progressive Migration for the Transmission Path", David Sarnoff Research Center (1993) and Puri, Atul, "Picture Format Scalable Coding for HDTV," ISO/IEC JTC1/SC29/WG11, MPEG 1993/390, Sydney, Australia (March 1993), discloses a technique that encodes a progressively scanned video with a method used for encoding an interlaced scanned video. In this method, the original progressive signal is converted to interlaced format, and both the interlaced signal and an enhancement signal (generated at the transmitter based on a comparison of the output of an interlace-to-progressive interpolator and the original progressive input) are transmitted. One objective of this method is to allow less expensive receivers to use only the interlaced signal and more expensive receivers to use the full progressive signal.